The improvement of current antiviral vaccines and the development of novel vaccines depends on improving our understanding of viral attachment and fusion glycoproteins. Critical insight into the structure and function of viral glycoproteins is provided by studying their biosynthesis in virus-infected cells. We have studied the biosynthesis of influenza virus hemagglutinin. The HA is a critical antigen for protecting individuals against influenza, which remains a major cause of morbidity and mortality nationally and internationally. In addition, the fact that the HA is the best characterized viral glycoprotein allows it to serve as a model for elucidating general features of viral glycoprotein structure, function, and antigenicity. In the past year we have pursued two ongoing studies of HA biosynthesis and transport. First, we have further characterized the site of HA assembly into trimers. Our findings suggest that, contrary to prevailing opinion, the HA trimerizes in a postendoplasmic reticulum (ER) compartment, probably the early portion of the Golgi complex (GC). Second, we have further characterized the effect of brefeldin A on HA assembly and transport. BFA is a fungal metabolite that interferes with the normal trafficking of vesicles between ER and GC, blocking the exocytosis of secretory and membrane proteins. Our findings suggest that in the presence of BFA, the ER is divided into subcompartments that can be distinguished by their accessibility to assembled HA molecules.